Chapter 2: Taking a closer look


Unpacking diversity: How to read a evolutionary tree


  • Depicts evolutionary relationships amongst a selected taxa
    • hypothetical
    • taxon (singular) can be species or groups


  • Taxa at ends of tree are usually living (extant)
    • taxa not at the end are extinct


  • Taxa are connected by branches, which join at nodes
    • node represents ancestor shared by 2+ taxa
    • branch length may or may not mean something


  • Infer degree of relationship based on nodes
    • 2 human virus are more closely related to one another that either is to bat virus.

Nodes in cladograms represent divergences or speciation events


Each node indicates the common ancestors of descendants
















Unique history (divergence or speciation) based represents key molecular change


Often represented as morphological trait in a fossil-based phylogeny

The molecular story of life


Change is the only constant


Evolution keep rolling, so species are not fixed in a hierarchy


Evolution keep rolling, so species are not fixed in a hierarchy


Change occurs across biological scales


  • Genes - Organisms - Species - Populations - Communities - Ecosystem - Biosphere


  • Are are interconnected and thus all are influenced by environmental change

Change occurs at molecular level


  • Once there is variation, there are two main processes that can work on that variation and lead to long term evolution
    • Natural Selection
    • Genetic Drift

Natural selection in the wild


Natural selection in the wild



White Sands formed recently (2,000 years old), and the substrate color of the white gypsum habitat contrasts dramatically with the surrounding Chihuahuan Desert’s dark soils habitat

The adaptive value of cryptic color



A very dramatic selection environment leading to concealing coloration for predation avoidance

Multiple colonizing lineages


Genetic drift


More gametes are produced than seed the next generation

Genetic drift


Random sampling gametes can change the genetic composition of a population over time

Genetic drift can have rapid effects during a “bottleneck


Genetic drift in the wild


Selection and Drift




  • Selection and drift are both always happening simultaneously
    • drift is by chance alone
    • selection is by reproductive success




  • But they are not always equally strong


  • Both processes can lead not only to change within populations but also divergence between populations

Selection and Speciation


Billions of years of speciation and extinction create biodiversity